Progesterone has been found to inhibit the onset of maternal behavior in the rat but its neural site of action is unknown. My previous research has shown that the medial preoptic area (MPOA) is involved in both the onset and maintenance of maternal behavior in the rat. The purpose of the proposed research is to extend my findings on the role of the MPOA in the neurohormonal mechanisms underlying maternal behavior by investigating the neural site at which progesterone acts to inhibit estrogen-induced maternal behavior in virgin female rats. Since the display of maternal behavior has been found to depend on the MPOA, a strong possibility is that progesterone has its inhibitory effects by interfering with the activational effects of the MPOA on maternal behavior. There are several possible ways in which this could occur: (1) Progesterone may act directly on the MPOA to decrease its activational influences on maternal behavior. (2) Progesterone may activate an extra-MPOA site which, in turn, inhibits the MPOA. (3) Progesterone may inhibit an extra-MPOA site which plays a stimulatory role in the control of maternal behavior via its connections with the MPOA. The proposed research, consisting of three experiments and employing both hormone implant techniques and lesioning techniques, is designed to answer these questions. In other words, information will be gained concerning both the neural site and mode of action of progesterone in inhibiting maternal behavior.